The subject matter of the present invention relates generally to continuously variable resistance attenuators of wide band frequency response and, in particular, to such an attenuator employing a lossy distortionless transmission line with its signal conductor providing an attenuation resistance uniformly distributed along such line which is engaged by the output contact of the attenuator for movement relative to change the attenuator setting. The lossy transmission line includes a plurality of separate shunt resistors of equal value connected between the signal conductor and a ground conductor and uniformly distributed along the transmission line. The shunt resistors have a total parallel value per unit length equal to the square of the characteristic impedance divided by the resistance per unit length of the signal conductor to provide the line with a uniform characteristic impedance. A termination resistor equal to the characteristic impedance of the line is connected to the output end thereof to prevent signal reflections.
The attenuator of the present invention has a greater high frequency response than previous continuously variable resistance attenuators including that shown in U.S. Pat. No. 3,157,846 of B. O. Weinschel. In addition it has a larger attenuation range within such frequency response. The present attenuator also has the advantage that it can be employed to terminate a transmission line since the lossy transmission line forming such attenuator has a constant input impedance equal to its characteristic impedance regardless of the attenuation setting of the output contact. Furthermore, the output impedance of the attenuator at such output contact is constant and equal to one-half of the characteristic impedance under such circumstances.
One embodiment of the attenuator of the present invention employs a movable lossy transmission line section and fixed input and output contacts so that the signal transit time between such contacts does not change with adjustments in the setting of the attenuator. In addition, this enables the size of the contacts to be minimized, thereby further increasing the high frequency response due to the resulting reduction of the inductance and capacitance associated with such contacts.
The present attenuator may be employed in place of any continuously variable resistance attenuator including potentiometers. However, the present attenuator is especially useful when employed in the vertical amplifier of a cathode ray oscilloscope having a wide band frequency response from DC to several hundred megahertz.
It is, therefore, one object of the present invention to provide improved continuously variable resistance attenuator having a wider frequency band width.
Another object of the invention is to provide such an attenuator in which a distortionless lossy transmission line is employed whose signal conductor forms an attenuation resistance uniformly distributed along its length and the output contact of the attenuator engages such signal conductor for movement relative thereto, and includes a plurality of separate shunt resistors uniformly spaced along the line to provide such line with a uniform characteristic impedance.
An additional object is to provide such an attenuator whose input impedance remains constant for different attenuation settings.
A further object is to provide such an attenuator having a constant output impedance at its output contact.
Still another object of the invention is to provide such an attenuator in which the lossy transmission line section is moved while the input contact and the output contact remain fixed in order to provide a constant signal transit time between such contacts in spite of changes in the attenuator setting.
A still further object of the invention is to provide such an attenuator with an extremely compact, rugged structure by providing the movable lossy transmission line as a resistance coating on a rotor plate of insulating material surrounded by a stator plate of insulating material on which the fixed input and output contacts are provided.